1. Field of the Invention
This invention relates to the formation of an epitaxial layer on a semiconductor wafer. More particularly, this invention relates to a process wherein slip and microcracking are inhibited during formation of an epitaxial layer on a semiconductor wafer.
2. Description of the Related Art
During the formation or growth of an epitaxial layer of semiconductor material on a semiconductor wafer, i.e., the growth of an epitaxial silicon layer on a single crystal silicon wafer, thermal stresses may occur in the wafer, during subsequent cooling of the wafer.
Conventionally a semiconductor wafer, during growth of an epitaxial layer thereon in a deposition chamber, rests on a support member or susceptor which usually comprises some other type of material, for example, a silicon carbide coated graphite material when the wafer is a silicon wafer. During the deposition of, for example, silicon on a silicon wafer resting on such a support, a pinning site or bridge of the growth material may form or deposit between the wafer and the support at a contact point between the wafer and support. When the wafer is in a vertical, or near vertical, position in the deposition chamber such a point contact may occur where the end edge of the wafer rests on the side-wall of the susceptor pocket.
When such a bridge or pinning site forms at this point during the deposition, subsequent cooling of the wafer and susceptor after the deposition may induce thermal stress in the wafer at this point because of the differences between the thermal coefficients of expansion of the wafer and susceptor.
This tendency to grow such a bridge, and the inducement of thermal stress in the wafer due to such occurrence, becomes particularly acute when thick epitaxial layers are grown on a semiconductor wafer, i.e., epitaxial layers of 50 .mu.m or more in thickness. If either the primary or a secondary flat on the wafer is at the bottom of the wafer, as it is positioned in the susceptor pocket, the problem of bridge formation may be exacerbated.
The thermal stress, which occurs as the wafer and susceptor cool, usually becomes great enough that the bridge fractures before ambient temperature is reached. However, evidence of the formation of the bridge can usually be detected by subsequent examination of the wafer under a microscope such as a Nomarski Phase Contrast microscope at about 20x.
Prior to the fracturing of the bridge, the thermal stress induced in the relatively thin wafer, at the point where the cooling wafer is connected to the susceptor through this bridge, may result in the occurrence of either microcracking or a slip or both in the wafer.
Microcracking, as shown in FIG. 1 at 2, is a small crack or fracture of the wafer which usually extends inwardly a short distance, e.g., about 1-10 millimeters (mm), from the point of thermal stress shown at 4 at the end edge of the wafer, although cracking across the entire diameter of the wafer is not unknown. Usually, the amount of thermal stress induced in the wafer by such bridge formation will not be sufficient to result in microcracking unless the thickness of the epitaxial layer exceeds about 50 micrometers (.mu.m).
Slip or slippage, illustrated at 8 in FIG. 2, may be defined as a fracture or disruption of the parallel planes of atoms, comprising epitaxial layer 12 on single crystal wafer 10, into adjoining planes separated by a fault line, i.e., adjacent atoms in the respective planes on both sides of the fault line are no longer coplanar. Although not illustrated, the slip may continue into the wafer itself, as well.
Such slippage and/or microcracking can result in rendering the wafer useless for subsequent processing to form integrated circuit structures; or at least result in the formation of defects in integrated circuit structures formed in the wafer in those regions where such slippage and/or micro-cracking has occurred.
It would, therefore, be desirable to eliminate this cause of thermal stress induced in the wafer, either by inhibiting formation of such a pinning site or bridge, or by removing the pinning site or bridge, after its formation, but prior to cooling of the wafer, i.e., prior to the inducement of thermal stress in the wafer by the existence of such a bridge.